Both polymethacrylate polymers, referred to herein as "PMAs", and olefin copolymers, referred to herein as "OCPs", are commercially available materials known to be useful as lubricating oil additives. PMAs possess better low temperature viscosity characteristics in lubricating oils than OCPs, and can be used to lower the pour point of lubricating oils, while OCPs cannot. OCPs, however, are more efficient thickeners for lubricating oils than PMAs and, therefore, can be used at lower concentrations in lubricating oils than PMAs to obtain the desired thickening effect.
Attempts have, therefore, been made to combine these two types of polymeric oil additives together to form a product having the beneficial performance properties of each type of additive. Physical blends of PMAs and OCPs in solution, at polymer solids levels of at least 30 weight percent, are not, however, useful as oil additives when either of these types of polymeric additives is present at a concentration of about 5 weight percent or higher. This is due to the thermodynamic incompatibility of PMAs and OCPs with each other in solutions, such as lubricating oils, when they are physically blended together in concentrated form. Incompatibility refers to the separation of the polymers into layers in a solvent, such as mineral oil, when the mixture is stored for a period of time. The separation of the additives into different phases prevents the effective use of the physical blend as a stable, viscosity index improver.
The theoretical aspects of compatible polymer mixtures are discussed in Heterogeneous Polymer Systems I. Polymer Oil-in-Oil Emulsions, G. E. Molau, Journal of Polymer Science, Part A, Vol. 3, pp 1267-1278 (1965); Copolymers, Polyblends and Composites, Advances in Chemistry Series 142, ACS, Chap. 7 (1975) entitled Compatibilization Concepts in Polymer Applications, N. G. Gaylord; and Polymer Blends, D. R. Paul and S. Newman, Academic Press, Vol. 2, Chap. 12 (1978), and other articles.
It is known that the presence of block, graft and/or block-graft copolymers, referred to herein in the aggregate simply as "graft copolymers", can be used to alleviate, to some degree, the problem of polymer incompatibility. However, attempts to chemically combine PMAs and OCPs to form stable, compatible polymer blend additives, have been only partially successful. The prior types of compatibilizers, a term used in the art to refer to materials that are useful to some degree to alleviate polymer incompatibility, formed by in situ polymerization techniques, as described in more detail below, are in reality mixtures of the graft, block and/or block-graft copolymers and homopolymers having various configurations and molecular weights. The term "compatibilizer" as used in the present invention refers to graft copolymer solutions prepared according to the process of the invention having polymer portions, referred to herein as "polymer segments", which are similar in chemical structure to the polymers to be blended therewith.
While the theoretical requirement that a graft copolymer compatibilizer should possess polymer segments similar in structure to each of the separately prepared polymers to be blended together to form a stable polymer blend is known, this requirement is not readily accomplished when separately prepared and commercially available PMAs and OCPs are blended together. Prior to the present invention, no process was known for graft polymerizing alkyl methacrylates onto separately prepared OCPs in solution to form a compatibilizer which is useful in minor amounts for blending major amounts of separately prepared and commercially available PMAs and OCPs to form a stable, high solids content polymer blend oil additive.
Although the theoretical possibility of preparing block or graft copolymers which will compatibilize separately prepared mixtures of polymers had been previously postulated, as in Compatibilization Concepts in Polymer Applications, supra, page 77, the practice of accomplishing such a result remains highly empirical. This is particularly the case when large amounts of commercially available polymers are to be compatibilized with small amounts of a copolymer compatibilizer. As mentioned above, the common commercial practice has been to form at least one of the polymers, which is desired to be present in the polymer mixture, from its monomers, such as the alkyl methacrylates, in the presence of a separately formed, second polymer such as the olefin copolymer. This in situ polymerization technique relies on the simultaneous production of a large amount of homopolymer of the first type (i.e. polyalkyl methacrylate) at the same time that a graft copolymer formed from monomers of this first type are graft polymerizing onto the second, separately prepared polymer (i.e. OCP). This in situ technique permits matching of the graft polymer segments of the first type polymer with the simultaneously prepared homopolymer of the first type. Since the in situ technique results in graft copolymers having polymer segments of the second type polymer, additonal minor amounts of separately prepared polymers of the second type (i.e. OCP) can be added to the in situ graft copolymer mixture without phase separation (incompatibility). This technique, however, requires polymerizing a large quantity of the monomers of the first type (i.e. alkyl methacrylates) to form a sufficient amount of the homopolymer of this type (i.e. PMA homopolymer) in situ, and does not contemplate the formation of a compatibilizer which can be used at small concentrations for blending larger amounts of commercially available, and separately prepared polymers of both types. The extreme difficulty in matching the polymer segments of a graft copolymer "compatibilizer" with separately prepared and commercially available polymers of both types, to be blended therewith, is believed to be the principal reason why prior workers have settled for the inefficient and unwieldy in situ processes and have avoided addressing the problem of forming a true compatibilizer.